There has been a steady increase in the incidence of diseases transmitted by food in the United States and growing concern over newly-emerging pathogens, notably Listeria monocytogenes. See D. L. Archer et al., Clin. Micro. Rev., 1, 377 (1988); J. M. Farber et al., Microbiol. Rev., 35, 476 (1991). L. monocytogenes is a ubiquitous, gram-positive, facultative anaerobic bacterium that is capable of growth over a wide temperature range (1.degree.-45.degree. C.), in high salt or nitrite concentrations, and at pH values between 4.8 and 9.6. See E. T. Ryser et al., Listeria, Listeriosis, and Food Safety; Marcel Dekker, Inc.: New York (1991). Although the first case of human listeriosis was reported over 60 years ago, it has only been in the last decade or so that L. monocytogenes has been firmly established as an important foodborne pathogen. During the 15 year period from 1973 through 1987, L. monocytogenes was second only to Salmonella in total deaths (70 and 88, respectively, of 274 total deaths) and first in death-to-case ratio (317 per 1,000 cases) in foodborne bacterial outbreaks of known outcome. See N. Bean et al., J. Food Prot., 53, 804 (1990). As evidenced by the apparent worldwide increases in the incidence and cases of food-related listeriosis, including a recent French outbreak, L. monocytogenes remains a serious threat to human health. For example, see A. A. Goulet et al., Bull. Epidemiol. Hebdomadaire, 4, 13 (1993).
Despite advances in the isolation, enumeration, and control of L. monocytogenes, less progress has been made to distinguish illness-causing isolates from harmless isolates at the molecular level. Definition and characterization of genes of interest, including virulence factors, can be an arduous task, even if cloning and mutagenesis systems are operational. For example, the gene probe disclosed by S. Notermans et al., Appl. Environ. Microbiol., 55, 902 (1989) was not specific for all L. monocytogenes. A typical commercially available detection assay for L. monocytogenes is based on a ribosomal RNA target-DNA probe and requires a critical mass of organism. As such, it is usually necessary to amplify the organism in culture before performing the assay. The process is further complicated, since associated phenotypes are difficult to select or score and/or all factors contributing to virulence have not been identified. For these reasons, a need exists for a method to reliably and rapidly differentiate virulent L. monocytogenes strains from otherwise phenotypically similar but avimlent varieties.